Building Semipermeable Films One Monomer at a Time: Structural Advantages via Molecular Layer Deposition vs Interfacial Polymerization

Molecular layer deposition (MLD) provides the opportunity to perform condensation polymerization one vaporized monomer at a time for the creation of precise, selective nanofilms for desalination membranes. Here, we compare the structure, chemistry, and morphology of two types of commercial interfacial polymerzation (IP) membranes with lab-made MLD films. M-phenylenediamine (MPD) and trimesoyl chloride (TMC) produced a cross-linked, aromatic polyamide often used in reverse osmosis membranes at MLD growth rates of 2.9 Å/cycle at 115 °C. Likewise, piperazine (PIP) and TMC formed polypiperazine amide, a common selective layer in nanofiltration membranes, with MLD growth rates of 1.5 Å/cycle at 115 °C. Ellipsometry and X-ray reflectivity results suggest that the surface of the MLD films is comprised of polymer segments roughly two monomers in length, which are connected at one end to the cross-linked bulk layer. As a result of this structure as well as the triple-functionality of TMC, MPD-TMC had a temperature window of stable growth rate from 115 to 150 °C, which is unlike any non-cross-linked MLD chemistries reported in the literature. Compared to IP films, corresponding MLD films were denser and morphologically conformal, which suggests a reduction in void volumes; this explains the high degree of salt rejection and reduced flux previously observed for exceptionally thin MPD-TMC MLD membranes. Using X-ray photoelectron spectroscopy and infrared spectroscopy, MLD PIP-TMC films evidenced a completely cross-linked internal structure, which lacked amine and carboxyl groups, pointing to a hydrophobic bulk structure, ideal for optimized water flux. Grazing-incidence wide-angle X-ray scattering showed broad features in each polyamide with d-spacings of 5.0 Å in PIP-TMC compared to that of 3.8 Å in MPD-TMC. While MLD and IP films were structurally identical to PIP-TMC, MPD-TMC IP films had a structure that may have been altered by post-treatment compared to MLD films. These results provide foundational insights into the MLD process, structure–performance relationships, and membrane fabrication.


A. Spatial Molecular Layer Deposition
Two substrates were used to analyze film growth and material characteristics.MPD-TMC MLD films grown at 130°C and 150°C were deposited on silicon substrates or metalized polyethylene naphthalate (PEN) (ROWO Coating).The metal coating was sputtered titanium (∽80 µm) which served as a reflective surface for ellipsometry.The narrow space between the inner and outer drums permitted use of silicon coupons with a thickness <400 µm and a width of ∽5 mm.
Precursors were stored in custom stainless-steel cylinders.Precursor flow was controlled by use of needle valves.Nitrogen flow was regulated with a mass flow controller (Alicat, MC-1SLPM-D) whose total flow was divided between the four nitrogen modules.
The pneumatic isolation valves (Fujikin) contained elastomer seals made from perfluoroalkoxy alkane (PFA) to prevent degradation in the presence of amines and acyl chlorides.
The pressure of the spatial reactor was monitored with capacitance monometers (121A Baratron®, MKS).One monomer was placed at each dosing line and a third measured the pressure of the drum.Precursor pressures are reported as the mean difference between the dose pressure and base pressure for the respective dosing line during reaction.
The inner drum was rotated about a shaft which was coupled to a stepper motor (MDrive23Plus, Schneider Electric).Speeds of up to 120 rpm were achieved.For a constant rotation speed, ω (rpm), residence time in each exposure zone (36°) was equal to 6/ω (s).The residence time in each purge zone (144°) was equal to 24/ω (s).The entire MLD system (excluding the pumps and exhaust) was placed inside a custom convection oven to ensure near isothermal conditions for the chamber, tubing, fittings, valves, and storage cylinders.Temperature was maintained with proportionalintegral (PI) control.To ensure steady state temperatures, heating and cooling times of at least 10, 12 and 24 h were performed before running MLD reactions at 115, 130 and 150°C, respectively.

B. X-Ray Photoelectron Spectroscopy Analysis
The XPS measurements of PIP-TMC (1.2 µm, 120 rpm, 115°C) and MPD-TMC (101 nm, 20 rpm, 115°C) samples provided elemental compositions which are shown in Figure S1 and summarized in Table S1.Carbon content levels above the stoichiometric values were attributed to adventitious species.A low presence of fluorine was attributed to contamination from the fluorocarbon vacuum grease used to lubricate the shaft seals of the reactor.The theoretical stoichiometric composition was calculated from the fully crosslinked polymer repeat units (Figure 6).

C. Infrared Spectroscopy Analysis
The following assignments were given to the PIP-TMC and MPD-TMC spectra shown in Figure 7. Amide I bands were assigned to MPD-TMC at 1650 cm -1 and PIP-TMC at 1620 cm -1 .The amide II band was assigned to MPD-TMC at 1517 cm -1 .Both polyamides showed amide III bands at 1278 cm -1 (PIP-TMC) and 1290 cm -1 (MPD-TMC).The shoulder of the MPD-TMC spectra at 715 cm -1 was assigned to the amide V mode.A weak, broad signal around 3200 cm -1 was ascribed to amide A & B stretching for MPD-TMC.The amide II, V, A, and B bands did not appear with PIP-TMC due to the lack of N-H groups.
The aromatic components of the polyamide led to several features in the FTIR spectra.A broad, weak peak around 3000 cm -1 (MPD-TMC) was assigned to aryl C-H stretches.Ring quadrant stretching could be seen in the MPD-TMC polyamide at 1598 cm -1 .For the PIP-TMC polyamide, the signal overlaps with the amide I band around 1595 cm -1 .Ring semi-circle stretch is a component of MPD, but does not appear in the TMC aromatic motif. 1 As such, a corresponding absorbance feature was found in MPD-TMC (1476 cm -1 ), but does not appear in PIP-TMC.For the MPD-TMC polyamide, the ring pucker (682 cm -1 ), and adjacent (775 cm -1 ) and lone (861 cm -1 ) C-H wags were assigned. 2e expected ring pucker and lone C-H wags were less discernable for the PIP-TMC polyamide and were not given assignment.

F. Surface Functional Group Spacing
The bulk mass density of the film (  ) and the  (growth per cycle) can be used to estimate the mass deposition rate,   (ng•nm -2 •cycle -1 ), the average number of amide bonds per volume (  , nm -3 ), and the average number of amide bonds deposited per MLD cycle (  , nm -2 •cycle -1 ).The bulk density is used rather than the top layer density because it represents the amount of material that propagates each MLD cycle.
Here,   is the molecular weight of a fully crosslinked repeat unit which contains six amide bonds (Figure 6).  is Avagadro's constant.The inverse of   , defined as  ̅  , represents the average area surrounding a single amide bond.Area density is used here as it is reasonable to assume that polymerization occurs at a flat, substrate surface plane at each MLD step since growth per cycle is on the order of the size of a polyamide monomer.

𝐴 ̅
To approximate the spacing between the amide bonds formed at each MLD cycle, a hexagonal array was used as shown in Figure S5.The array is used as a simplification, not a suggestion of crystallinity.This arrangement assumes that any segmental motion of polymer tails (in the loose, top stratum) is anchored to the points of the array as constrained by the crosslinked bulk layer.By equating  ̅  to the unit area of the array containing an equivalent of one amide bond, the average distance between each amide bond, , may be calculated.
Unit area geometry for a 2D hexagonal array, used as an approximate layout for functional end groups during MLD.

G. X-ray Scattering Plots
In addition to the as-received commercial IP membranes and the MLD films presented in the main text, we also investigated the effect of rinsing the membranes (for preservative removal) on the structure of the commercial membranes.After rinsing the commercial MPD-TMC membrane, a small shift of the scattering feature at ∽1.3 Å -1 towards higher q was observed (Figure S6a), corresponding to a decrease in the molecular packing distances and giving rise to this peak.Interestingly, the change in the q position of this (5) (6) feature (proposed to be due to post-processing) is subtle compared to the shift to higher q observed for scattering features associated with small packing distances (>1.3 Å -1 ).As shown in Figure S6b, minimal changes are observed in the PIP-TMC commercial membranes following rinsing.

Figure S3 .
Figure S3.XRR data and respective fits for each sample.

Figure S4 .
Figure S4.Scattering length density (SLD) model profiles used to generate XRR model curves for each sample.The profile without roughness is shown as an aid for visualization of strata within the sample.

Figure S6 .
Figure S6.1D integrated radial profile of the as-received and rinsed commercial IP membranes compared to films prepared by MLD for a) MPD-TMC and b) PIP-TMCchemistries.These data are normalized to the peak maximum between 1.0-2.0Å -1 for easy visualization of changes in peak position and shape.

Figure S7 .
Figure S7.(a), (c), and (e) q xy vs q z scattering plots and (b), (d), and (f) variations of scattering in chi over angles of 5-85°, taken in 10° slices for MPD-TMC films.(a) and (b) are measurements of the as-received commercial IP MPD-TMC TFC membrane (FilmTec XLE).(c) and (d) are measurements of the rinsed commercial IP MPD-TMC TFC membrane.(e) and (f) are measurements of a MLD MPD-TMC 180 nm film prepared at115ºC.These data were intensity-scaled by |ℎ|, background-subtracted, and normalized to the peak maximum (between 1.0-2.0Å -1 ) when integrated over all angles.Note for (c) and (e), that single images are shown and analyzed due to difficulties in combining two images to account for the gaps in the detector.

Figure S8 .
Figure S8.(a), (c), and (e) q xy vs q z scattering plots and (b), (d), and (f) variations of scattering in chi over angles of 5-85°, taken in 10° slices for PIP-TMC films.(a) and (b) are measurements of the as-received commercial IP PIP-TMC TFC membrane (FilmTec NF270).(c) and (d) are measurements of the rinsed commercial IP PIP-TMC TFC membrane.(e) and (f) are measurements of the MLD PIP-TMC 64 nm film prepared at115ºC.These data were intensity-scaled by |ℎ|, background-subtracted, and normalized to the peak maximum (between 1.0-2.0Å -1 ) when integrated over all angles.* highlights a scattering artifact in both the 2D and 1D data.

Table S2 .
A Summary of XRR Results for MLD Polyamide Films